The present invention relates to a method for producing a copper-clad laminate.
Conventionally, as means for attaining interlayer electrical connection between inner circuit layers contained in multilayer printed wiring boards, there have been employed, in an inner circuit layer substrate, means such as an interstitial via hole (IVH)xe2x80x94a type of through holexe2x80x94or a blind via hole (BVH)xe2x80x94a cavity hole. These interlayer connection means, which enable formation of fine circuits of printed wiring boards, have been generally employed over a wide range as means for attaining high-density packaging, or mounting of devices.
A copper-clad laminate or a printed wiring board having such interlayer connection means as an interstitial via hole (IVH) or a blind via hole (BVH) is produced through a buildup process in which copper foil layers are sequentially bonded to an outer layer of a printed wiring boardxe2x80x94serving as core materialxe2x80x94in which formation of an inner circuit has been completed, to thereby form a copper-foil circuit layer comprising multilayers and a variety of via holes are buried within a substrate. FIG. 9 shows a method of pressing outer copper foil layers in a build-up process for producing a copper-clad laminate having an interstitial via hole (IVH)xe2x80x94a through holexe2x80x94and a blind via hole (BVH)xe2x80x94a cavity.
As shown in FIG. 9, in general, a through hole serving as an interstitial via hole (IVH) and a cavity serving as a blind via hole (BVH) are not filled with any matter. Thus, when a typical resin-coated copper foil is pressed in a manner as shown in FIG. 9, coated resin used for bonding an outer copper foil layer flows into the holes, to thereby fill the holes.
However, such resin has a thermal expansion coefficient as large as approximately ten times that of copper foilxe2x80x94metallic material. Thus, during a cooling step involving curing of the resin after press-forming at high temperature, the degree of shrinkage of the resin is also greater than that of substrate. As a result, the copper foil bonded to the resin is pulled toward the inside of the holes due to shrinkage force of the resin.
In addition, in recent years, the requirement for forming fine circuits has led to a tendency of reduction in thickness of an outer copper foil layer. Accordingly, the outer copper foil layer is pulled toward the substrate, to thereby form a dent at the pulled area, and a surface of a copper-clad laminate produced from such a copper foil layer may fail to remain flat. Hereinafter, the dented state is referred to as xe2x80x9ca dent faultxe2x80x9d (persons in the art may call the state xe2x80x9ca dimple fault.xe2x80x9d). FIG. 10 shows a dent fault actually occurring in an outer copper foil layer above an interstitial via hole (IVH).
When the outer copper foil layer is partially dented as shown in FIG. 10, sufficient adhesion may fail to be maintained between the dented portion of the outer copper foil layer and an etching resist layer at a stage when the etching resist layer such as dry film is formed on the outer copper foil layer, exposed, and developed, so as to form an external circuit. If such a resist adhesion failure occurs, during etching of the copper foil circuit only the portion of a copper foil circuit corresponding to the adhesion failure becomes finer, as compared with the target circuit width, and in an serious case, the portion is completely dissolved, to thereby cause an open circuit. In other words, etching precision of the portions of the outer copper foil layer immediately above the formed interstitial via hole (IVH) or blind via hole (BVH) is deteriorated, to thereby lower the production yield considerably.